Not applicable.
This invention relates to bearing caps, and in particular to main bearing caps for the crankshaft of an internal combustion engine.
The introduction of main bearing caps made by powder metallurgy (P/M) into American automobiles has resulted in substantial economies for the industry. Substantial cost savings arise from the reduction in metal removal operations (broaching, milling, drilling and sawing) that were needed in the previously used cast iron castings. This is because the P/M process produces a much more precise form than is possible by the casting process. The capital cost associated with a machining line for iron castings has historically been in the 5 to 10 million dollar range, depending upon complexity and production output required. By contrast, the cost of a line to machine a comparable P/M pre-form has been in the range of 1 to 2 million dollars.
In an ongoing effort to further reduce the investment costs of manufacturing main bearing caps, research and development programs have been dedicated toward elimination of the 1 to 2 million dollars per engine program that is still needed for machining the P/M main bearing caps. One successful program resulted in the xe2x80x9cintegral dowelxe2x80x9d which is taught in International Patent Publication No. WO 97/42424 (International Application No. PCT/US97/04050 filed Mar. 12, 1997; U.S. Pat. No. 6,086,258 issued Jul. 11, 2000), the disclosure of which is hereby incorporated by reference. This feature eliminates two or three milling operations which may approximately halve the investment needed, leaving drilling and tapping of side-bolt 8 holes as the major remaining machining process (see FIG. 1).
It is not possible with conventional or known P/M technology to compact a horizontal threaded hole since such a hole is at right angles to the direction of powder compaction and the tool element producing this feature would prevent the compact from being removed from the die cavity. Many efforts have failed in attempts to form horizontal threaded holes of the precision required by main bearing caps. Therefore, an alternative solution was sought and discovered by the inventors and is the basis for this patent application.
The need for the side-bolts (FIG. 1) is associated with the customer preference for a xe2x80x9cquietxe2x80x9d vehicle when in the driver/passenger compartment. One significant contributor to unpleasant noise (termed noise, vibration and harshness, or NVH, in the automotive industry) is the transmission of vibrations which emanate from the engine crankshaft. The high speed revolution and the vibrational stresses it transmits to the main bearings and the main bearing caps, which support and retain them, is a particularly persistent source of NVH. A design solution that was made popular in the last 4 to 5 years is to stiffen the support of the crankshaft by bolting the main bearing caps to the side of the cylinder block, thereby forming a stiff cage or box structure. This has shown significant reductions in NVH perception in a number of vehicles.
The downside of this solution is two-fold. Firstly, the cost of the side drilling and tapping of the P/M bearing cap and the side drilling of the engine block adds to vehicle cost. Secondly, the side holes through the engine block are potential oil leak paths which require special bolts, some with a gasket material incorporated to ensure a leak-free engine.
The concept of vertical side-bolts or inclined angle side-bolts is not new, however, the technology to date has had several drawbacks when applied to a cast iron main bearing cap. Firstly, the holes still have to be drilled in the casting and the rough cast surface has to be machined to provide a flat face for the bolt-head to bear against. Secondly, the xe2x80x9cunderarmxe2x80x9d adjoining face 14 (see FIG. 4A, which shows two holes through the foot-one, two or more may be provided) has to be precisely machined to match the height differential between the joint face 16 and the arm support face 32 of the cylinder block to avoid leaving an excessive gap (g). This degree of sophistication has prevented the adoption of the vertical or inclined side-bolts in mass production.
A third drawback is that a flat-faced bolted joint is not rigid in the plane 3 of the joint face (See FIG. 5A). That means that there can be a sliding action between the bolted faces. This sliding action can be a very small length and can be cyclic, in time with the rotational speed of the engine. In this case, the phenomenon is known as xe2x80x9cfretting wearxe2x80x9d, which results in bolted surfaces wearing by a mechanism that is not fully understood according to even the most recent accounts. It involves surface oxidation, localized pitting and when severe, can lead to surface cracks and even section fracture. There are some methods available to reduce the severity of fretting wear, and the most effective appears to be the use of a fitted dowel 5, which provides a precision location between the two surfaces and minimizes sliding vibration (FIG. 5B). Unfortunately, precision dowels are costly and require great precision in the holes that are drilled in the two mating components. This is not a cost-effective option for high volume, low cost engine manufacture. The use of integral dowels 7 in a P/M main bearing cap in the xe2x80x9carmxe2x80x9d sections is an alternative solution (as taught in International Patent Publication No. WO 97/42424) (FIGS. 5C and 5D), but the problem of the height differential precision persists.
A solution to these problems is offered by this invention, which alters the direction of the bolt holes to either vertical (FIG. 2) or at an inclination (FIG. 3) such that the holes in the P/M bearing cap can be compacted instead of drilled and tapped or milled. This also avoids the need to breach the cylinder block wall, thereby eliminating any risk of oil leakage from such holes.
To deal with the xe2x80x9carmxe2x80x9d height differential, and also the joint face fretting wear, a design was conceived that is only possible to produce in large volumes by powder metallurgy. This consists of an integral dowel at least partially around the bolt hole on the underarm surface of the bearing cap, with an array of fine raised pointed conical teeth at least partially around the dowel that bite into the cylinder block facing surface when bolt pressure is applied. As the bolt is tightened down, the xe2x80x9cteethxe2x80x9d both bite into the cylinder block material and also compress themselves. In the case of an aluminum alloy cylinder block, there is more indentation than tooth compression. In the case of a cast iron cylinder block, there is less indentation and more tooth compression. However, in both cases the teeth provide a fretting resistant bolted joint, and simultaneously provide an accommodation for the variation in height differentials on the block and on the P/M bearing cap. This is accomplished by the simple fact that a mismatch in height differentials that leaves a larger than average underarm gap is accommodated by the teeth not indenting or compressing as much as when there is a smaller than average gap.
In another aspect, a splayed or bow-tie joint can be made, with the side bolts threaded into the block at an oblique (non-perpendicular) angle by making the bolt holes elongated, with the bolt head supporting surfaces of the bearing cap arms at an oblique angle to the hole. The bolt therefore extends at an angle relative to the sides of the hole, which are straight and parallel to the sides of the bearing cap and to the main bolt holes in the bearing cap.
In a method of assembling bearing caps of the invention, the bearing caps are first pressed into position with a shaped punch that ensure positive indentation of the teeth into the material of the block and insertion of the integral dowels into the counterbores of the block, followed by bolting the main bearing cap to the cylinder block, so as to produce more consistent bearing bores defined between the bearing cap and the block.
These and other objects and advantages of the invention will be apparent from the detailed description and drawings.